Extreme ultraviolet lithography process and mask

ABSTRACT

An apparatus comprises a low EUV reflectivity (LEUVR) mask. The LEUVR mask includes a low thermal expansion material (LTEM) layer; a reflective multilayer (ML) over the LTEM layer; and a patterned absorption layer over the reflective ML. The reflective ML has less than 2% EUV reflectivity.

This application is a divisional of U.S. Ser. No. 14/020,302 entitled“An Extreme Ultraviolet Lithography Process and Mask,” filed Sep. 6,2013, herein incorporated by reference in its entirety.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced rapidgrowth in the past several decades. Technological advances insemiconductor materials and design have produced increasingly smallerand more complex circuits. These material and design advances have beenmade possible as the technologies related to processing andmanufacturing have also undergone technical advances. As the size of adevice feature, such as gate length, has decreased, numerous challengeshave risen. High resolution lithography processes are often one of themore important areas to decreasing feature size, and improvements inthis area are generally desired. One lithography technique is extremeultraviolet (EUV) lithography. Other techniques include X-Raylithography, ion beam projection lithography, electron beam projectionlithography, and multiple electron beam maskless lithography.

EUV lithography is a promising patterning technology for semiconductortechnology nodes with very small feature sizes, such as 14-nm, andbeyond. EUV lithography is very similar to optical lithography in thatit uses a mask to print wafers. However, unlike optical lithography, EUVemploys light in the EUV region, e.g., at about 13.5 nm. At thewavelength of 13.5 nm, most materials are highly absorbing. Thus,reflective optics, rather than refractive optics, are commonly used inEUV lithography. Although existing methods of EUV lithography have beengenerally adequate for their intended purposes, they have not beenentirely satisfactory in all respects. For example, the EUV lightproduced by tin plasma, such as DPP (discharge-produced plasma) and LPP(laser-produced plasma), emits some out of band (OOB) radiation. Aportion of the OOB radiation (sometimes referred to as a flare) can alsoarrive at the target substrate (e.g., a wafer) and cause image contrastloss. So it is desired to have further improvements in this area.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a block diagram of a lithography process for implementing oneor more embodiments of the present disclosure.

FIG. 2 is a diagrammatic cross-sectional of a blank mask at variousstages of a lithography process constructed according to aspects of thepresent disclosure.

FIG. 3 is a diagrammatic cross-sectional view of various aspects of oneembodiment of an EUV mask at various stages of a lithography processconstructed according to aspects of the present disclosure.

FIGS. 4A and 4B are diagrammatic cross-sectional views of another EUVmask at various stages of a lithography process constructed according toaspects of the present disclosure.

FIG. 5 is a flow chart of an example method for evaluating deepultraviolet (DUV) flare impact according to various aspects of thepresent disclosure.

FIGS. 6A and 6B are top schematic views of patterning a substrate usingdifferent masks in the method of FIG. 5 according to various aspects ofthe present disclosure.

FIG. 7 is a chart of critical dimension (CD) vs. DUV flare according tovarious aspects of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and dose not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath”, “below”, “lower”,“above”, “upper”, “over” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. For example, if the device in the figures is turned over,elements described as being “below” or “beneath” other elements orfeatures would then be oriented “above” the other elements or features.Thus, the exemplary term “below” can encompass both an orientation ofabove and below. The apparatus may be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein may likewise be interpreted accordingly.

Referring to FIG. 1, an EUV lithography process 10 that may benefit fromone or more embodiments of the present invention is disclosed. The EUVlithography process 10 employs an EUV radiation source 20, including anEUV wavelength of about 13.5 nm.

The EUV lithography process 10 also employs an illuminator 30. Theilluminator 30 may comprise reflective optics, such as a single mirroror a mirror system having multiple mirrors in order to direct light fromthe radiation source 20 onto a mask 40. In the EUV wavelength range,reflective optics is employed generally. Refractive optics, however, canalso be realized by zoneplates.

The EUV lithography process 10 also employs a mask 40 (the terms mask,photomask, and reticle are used herein to refer to the same item), ormultiple masks. In the present embodiment, the mask 40 is a reflectivemask. The mask 40 may incorporate other resolution enhancementtechniques such as optical proximity correction (OPC). The mask 40 willbe described in further detail later.

The EUV lithography system and process 10 also employs a projectionoptics box (POB) 50. The POB 50 may have refractive optics or reflectiveoptics. The radiation reflected from the mask 40 (e.g., a patternedradiation) is collected by the POB 50.

The target 60 includes a semiconductor wafer with a photosensitive layer(e.g., photoresist layer or resist), which is sensitive to the EUVradiation. The target 60 may be held by a target substrate stage. Thetarget substrate stage provides control of the target substrate positionsuch that the image of the mask is scanned onto the target substrate ina repetitive fashion (though other lithography methods are possible).

The EUV exposure light source may contain some of out of band radiation(OOB) and a part of the radiation can arrival wafer surface (sometimesreferred to as a flare) and causes image contrast loss. Comparing withEUV, the OOB which can arrive wafer surface may have longer wavelength,such as deep ultraviolet (DUV) wavelength. So a stray light level of DUVflare should be much lower than that of EUV. In this situation, themajor DUV flare contribution may come from in situ rather than proximityregion. In EUV lithography process, it is important to evaluate the DUVflare impacts for better optical simulation, prediction and making astrategy to suppress this impact. Due to DUV flare in EUV scanner may bea local flare rather than stray light caused by non-pure illuminationwavelength and impacts of DUV flare may depend on mask structure andpattern density, a method of evaluating this local flare anddistinguishing a contribution between EUV and DUV flare is described asbelow.

The following description refers to the mask 40 and mask fabricationprocess. The mask fabrication process usually includes two steps: a masksubstrate fabrication process and a mask patterning process. A masksubstrate is formed by a stack of layers (e.g., multiple reflectivelayers). The mask substrate is patterned during the mask patterningprocess to have a design of a layer of an integrated circuit (IC) device(or chip). The patterned mask is then used to transfer circuit patterns(e.g., the design of a layer of an IC device) onto a semiconductorwafer. The patterns can be transferred over and over onto multiplewafers through various lithography processes. Several masks (forexample, a set of more than 50 masks) may be used to construct acomplete IC device.

Referring to FIG. 2, a mask substrate 100 includes a material layer 110made of low thermal expansion material (LTEM). The LTEM materialincludes TiO₂, doped SiO₂, and/or other low thermal expansion materialsknown in the art. The LTEM layer 110 serves to minimize image distortiondue to mask heating. In the present embodiment, the LTEM layer 110includes materials with a low defect level and a smooth surface. Inaddition, a conductive layer 105 may be deposited under (as shown in thefigure) the LTEM layer 110 for electrostatic chucking the mask. In anembodiment, the conductive layer 105 includes chromium nitride (CrN),though other compositions are possible.

The mask substrate 100 also includes a first reflective multilayer (ML)120 deposited over the LTEM layer 110. According to Fresnel equations,light reflection will occur when light propagates across the interfacebetween two materials of different refractive indices. The reflectedlight is larger when the difference of refractive indices is larger. Toincrease the reflected light, one may also increase the number ofinterfaces by deposing a multilayer of alternating materials and letlight reflected from different interfaces interfere constructively bychoosing appropriate thickness for each layer inside the multilayer.However, the absorption of the employed materials for the multilayerlimits the highest reflectivity that can be achieved. The firstreflective ML 120 includes a plurality of film pairs, such asmolybdenum-silicon (Mo/Si) film pairs (e.g., a layer of molybdenum aboveor below a layer of silicon in each film pair). Alternatively, the firstreflective ML 120 may include molybdenum-beryllium (Mo/Be) film pairs,or any material pair that is highly reflective at EUV wavelengths can beutilized for the first reflective ML 120. The thickness of each layer ofthe first reflective ML 120 depends on the EUV wavelength and theincident angle. The thickness of the first reflective ML 120 is adjustedto achieve a maximum constructive interference of the EUV lightreflected at each interface and a minimum absorption of the EUV light bythe first reflective ML 120. The first reflective ML 120 may be selectedsuch that it provides a high reflectivity to a selected radiationtype/wavelength. A typical number of film pairs is 20-80, however anynumber of film pairs is possible. The first reflective ML 120 usuallyachieves a reflectance of 0.65 or above. In an embodiment, the firstreflective ML 120 includes forty pairs of layers of Mo/Si. Each Mo/Sifilm pair has a thickness of about 7 nm, with a total thickness of 280nm. In this case, a reflectivity of about 70% is achieved. In oneembodiment, the first ML 120 is configured as forty pairs of films of 3nm Mo and 4 nm Si.

The mask substrate 100 may also include a capping layer 130 disposedabove the first reflective ML 120 to prevent oxidation of the reflectiveML. In one embodiment, the capping layer 130 includes ruthenium (Ru), Rucompounds such as RuB, RuSi, chromium (Cr), Cr oxide, and Cr nitride.The capping layer 130 has a thickness of about 2.5 nm.

The mask substrate 100 also includes an absorption layer 140 formedabove the capping layer 130. The absorption layer 140 includes multiplefilm layers with each film containing chromium, chromium oxide, chromiumnitride, titanium, titanium oxide, titanium nitride, tantalum, tantalumoxide, tantalum nitride, tantalum oxynitride, tantalum boron nitride,tantalum boron oxide, tantalum boron oxynitride, aluminum,aluminum-copper, aluminum oxide, silver, silver oxide, palladium,ruthenium, molybdenum, other suitable materials, or mixture of some ofthe above. In one embodiment, the absorption layer 140 includes 70 nmtantalum boron nitride (TaBN). In another embodiment, the absorptionlayer 140 includes 56 nm tantalum boron nitride (TaBN) and 14 nmtantalum boron oxide (TaBO) deposited over the TaBN layer.

One or more of the layers 105, 120, 130 and 140 may be formed by variousmethods, including physical vapor deposition (PVD) process such asevaporation and DC magnetron sputtering, a plating process such aselectrode-less plating or electroplating, a chemical vapor deposition(CVD) process such as atmospheric pressure CVD (APCVD), low pressure CVD(LPCVD), plasma enhanced CVD (PECVD), or high density plasma CVD (HDPCVD), ion beam deposition, spin-on coating, metal-organic decomposition(MOD), and/or other methods known in the art.

Referring to FIG. 3, the absorption layer 140 is patterned to form thedesign layout pattern EUV mask 200 having first and second region, 210and 220. In the first region 210, the absorption layer 140 remains whilein the second region 220, it is removed. The absorption layer 140 can bepatterned by various patterning techniques. One such technique includesusing a resist coating (e.g., spin-on coating), soft baking, maskaligning, exposure, post-exposure baking, developing the resist,rinsing, and drying (e.g., hard baking). An etching process follows toremove the absorption layer 140 in the second region 220. The etchingprocess may include dry (plasma) etching, wet etching, and/or otheretching methods. For example, the dry etching process may implement afluorine-containing gas (e.g., CF₄, SF₆, CH₂F₂, CHF₃, and/or C₂F₆),chlorine-containing gas (e.g., Cl₂, CHCl₃, CCl₄, and/or BCl₃),bromine-containing gas (e.g., HBr and/or CHBR₃), iodine-containing gas,other suitable gases and/or plasmas, and/or combinations thereof.Alternative patterning processes include maskless photolithography,electron-beam writing, direct-writing, and/or ion-beam writing.

Referring to FIG. 4A, a low EUV reflectivity (LEUVR) mask 400 is formedsimilarly in many respects to the EUV mask 200 discussed above in FIG.3, except it has a different ML, a second ML 320. The second ML 320 isconfigured to have a low EUV reflectivity. For example, the reflectivityof the second ML 320 is less than 2%. In one embodiment, the second ML320 is configured as forty pairs of films of 1.5 nm Mo and 2 nm Si. Inanother embodiment, the second ML 320 is configured as 280 nm MoSi. Inyet another embodiment, the second ML 320 is configured as forty pairsof films of 4.5 nm Mo and 6 nm Si.

Alternatively, the LEUVR mask 400 is configured to have the second ML320 of forty film pairs of 1.5 nm Mo and 2 nm Si. In the embodiment ofFIG. 4B, the LEUVR mask 400 has no capping layer (such as the layer 130shown in FIG. 4A).

The EUV masks 200 and the LEUVR mask 400 may also incorporate otherresolution enhancement techniques such as an optical proximitycorrection (OPC). The mask 200 may undergo a defect repair process by amask defect repair system. The mask defect repair system is a suitablesystem, such as an e-beam repair system and/or a focused ion beam (FIB)repair system.

FIG. 5 is a flowchart of a method 500 of evaluating DUV flare impactsconstructed according to aspects of the present disclosure. FIGS. 6A and6B are diagrammatic top views of patterning a target 600 at variousstages of the method 500.

The method 500 begins at step 502 by providing the EUV mask 200 having afirst EUV reflectivity r₁ and the LEUVR mask 400 having a second EUVreflectivity r₂. The EUV mask 200 and the LEUVR mask 400 has a sameabsorber pattern but different ML configuration to produce different EUVreflectivity, as described in FIG. 4A.

Referring FIGS. 5 and 6A, the method 500 proceeds to step 504 byperforming a first exposure process using the EUV mask 200 on asubstrate 600 by an EUV scanner. In present embodiment, the EUV scanneremploys an EUV radiation carrying an OOB radiation. The first exposureprocess starts at a first region 601 in the substrate 600 coated with aphotoresist layer, then a second exposure process is performed to asecond region 602, a third exposure process is performed in a thirdregion 603, and so on. In the present embodiment, the first exposureprocess is conducted according to a first exposure dose matrix. Thefirst exposure dose matrix is configured such that, through the EUV mask200, using an exposure dose E₁₁ to expose the first region 601, using anexposure dose E₁₂, which is equal to E₁₁−Δ to expose the second region602 (here Δ=r₂/r₁×E₁₁); using an exposure dose E₁₃, which is equal toE₁₁−2Δ to expose the third region 603; . . . , and using an exposuredose E_(1N), which is equal to E₁₁−(N−1) Δ to expose the N^(th) region60N.

In one embodiment, the E₁₁ is an optimized exposure dose (Eop) for theEUV mask 200. The Eop may be determined based on an exposure dose forthe pattern of the EUV mask 200 to achieve a pre-specified targetdimension on the substrate 600 under a corresponding single exposureprocess. The Eop may vary according to pattern density of the EUV mask200. Thus E₁₂ is equal to Eop−Δ, E₁₃ is equal to Eop−2Δ . . . E_(1N) isequal to Eop−(N−1)Δ.

With the first EUV reflectivity r₁ of the EUV mask 200, an exposure doseE_(1N) received by the region 60N of the substrate 600 is aboutr₁×E₁₁−r₂(N−1) E₁₁. When the exposure dose has both of EUV dose and OOBflare does, the E_(1N) is about r₁×E_(11EUV)−r₂(N−1) E_(11EUV) plusE_(11OOB)−(N−1) (r₂/r₁) E_(11OOB), here E_(11EUV) represents an EUV doseportion of E₁₁ and E_(11OOB) represents an OOB flare dose portion ofE₁₁. In present embodiment, considering the OOB flare dose E_(11DUV) issubstantial smaller than the EUV dose E_(11EUV) in the EUV scanner andr₂ is substantial smaller than r₁, it may be a fair estimation that theexposure dose E_(1N) received by the region 60N receives is close tor₁×E_(11EUV)−r₂(N−1) E_(11EUV) plus E_(11OOB). As an example, when theexposure dose has EUV dose and DUV flare does, the region 601 mayreceive the exposure dose as r₁×E_(11EUV) plus E_(11DUV); the region 602may receive exposure dose as r₁×E_(11EUV)−r₂×E_(11EUV) plus E_(11DUV);the region 603 may receive exposure dose as E_(11EUV)−2r₂×E_(11EUV) plusE_(11DUV); . . . , the region 60N may receive exposure dose asr₁×E_(11EUV)−r₂(N−1)E_(11EUV) and E_(11DUV).

Referring FIGS. 5 and 6B, the method 500 proceeds to step 506 byperforming second exposure using the LEUVR mask 400 on the substrate 600by the same EUV scanner having the same radiation. In the presentembodiment, the second exposure process is conducted according to asecond exposure dose matrix. The second exposure dose matrix isconfigured such that, through the LEUVR mask 400, using zero exposuredose to expose the first region 601, using an exposure dose E₂₂, whichis equal to E₁₁ to expose the second region 602; using an exposure doseE₂₃, which is equal to 2E₁₁ to expose the third region 603 . . . , andusing an exposure dose E_(2N), which is equal to (N−1) E₁₁ to expose theN^(th) region 60N.

With the second EUV reflectivity r₂ of the LEUVR mask 400, the exposuredose received by the N^(th) region 60N is close to r₂×(N−1)E_(11EUV) and(N−1)E_(11OOB). As an example, the region 601 receives zero dose; theregion 602 receives exposure dose as r₂×E_(11EUV) and E_(11DUV); theregion 603 may receive exposure dose as 2r₂×E_(11EUV) and 2E_(11DUV); .. . , the region 60N may receive exposure dose as r₂(N−1)E_(11EUV) and(N−1)E_(11DUV). In present embodiment, considering r₂ is substantialsmall, it may be a fair estimation that exposure does received by theregion 60N is close to r₂(N−1)E_(11EUV) plus (N−1) E_(11OOB).

Thus after the first and second exposure processes, a total exposed doseE_(T) received by each region of the substrate 600 is close to a sum ofEUV dose and OOB flare dose received in these two exposures through thetwo masks, respectively. Considering the OOB flare dose is substantialsmaller than the EUV dose in the EUV scanner and r₂ is substantialsmaller than r₁, it may be a fair estimation that E_(T1) received by theregion 601 is close to r₁×E_(11EUV)+E_(11OOB); E_(T2) received by region602 is close to r₁×E_(11EUV)+2E_(11OOB); E_(T3) received by the region603 is close to r₁×E_(11EUV)+3E_(11OOB); . . . ; E_(TN) received by theregion 60N is close to r₁×E_(11EUV)+N×E_(11OOB). For example, when theexposure dose has EUV dose and DUV flare does, E_(TN) received by theregion 60N is close to r₁×E_(11EUV)+N×E_(11DUV). In another word, eachregion in the substrate 600 receives a substantial same EUV exposuredose, r₁×E_(11EUV), and a different OOB flare dose, N×E_(11OOB).

Referring to FIG. 5, the method 500 proceeds to step 508 by obtainingcritical dimension (CD) data for each region of the substrate 600. Afterperforming the first and the second exposure process, a developingprocess is performed on the photoresist layer of the substrate 600.During the developing process, a developing solution is applied to thephotoresist layer. In an example, the developing solution is a basicsolution, such as tetramethylammonium hydroxide (TMAH). The developingsolution removes exposed or unexposed portions of the photoresist layerdepending on the resist material. For example, the photoresist layerincludes positive-tone resist material, so the developing processremoves (dissolves) the exposed portions of the photoresist layer andleave the unexposed portions of the photoresist layer over the substrate600. Alternatively, where the photoresist layer includes negative-toneresist material, the developing process removes (dissolves) theunexposed portions of the photoresist layer and leave the exposedportions of the photoresist layer over the substrate 600. A rinsingprocess, such as a de-ionized (DI) water rinse. The rinsing process mayremove residue particles.

A CD measurement is performed to obtain CD₁ of the first region 601, CD₂of the second region 602, CD₃ of the second region 603, . . . . CD_(N)of the second region 60N. A relationship between each CD_(N) and acorresponding total exposure dose E_(T) may be studied by any suitablemethod to evaluate DUV flare impacts, produced by the EUV scanner, onthe EUV mask 200 (with its mask structure and pattern density). As anexample, a plot of CD vs. total exposure dose E_(T) is formed with totalexposure dose E_(T) as its X-axis and CD as its Y-axis. A trend line ofCD vs. total exposure dose E_(T) may be obtained as well from the plot.Since the EUV exposure dose received by each region in the substrate 600is substantially same and variations of the total exposure dose E_(T)represent mainly variations of DUV flare, the difference between Y-axisintercept and CD₁ is proportional to DUV level, as shown in FIG. 7.

Additional steps can be provided before, during, and after the method500, and some of the steps described can be replaced, eliminated, ormoved around for additional embodiments of the method 500.

Based on the above, the present disclosure offers the EUV lithographyprocess to evaluate DUV flare impact on an EUV mask exposed by an EUVscanner. The process employs a mask pair having same pattern butdifferent EUV reflectivity. The process also employs two exposure dosematrixes for multiple exposure processes using the mask pair to revealDUV flare impact on the EUV mask by the EUV scanner.

The present disclosure is directed towards lithography systems andprocesses. In one embodiment, an extreme ultraviolet lithography (EUVL)process includes receiving a mask pair having a same pattern. The maskpair includes an extreme ultraviolet (EUV) mask having a first EUVreflectivity r₁ and a low EUV reflectivity mask having a second EUVreflectivity r₂. The process also includes receiving a substrate coatedwith a photoresist layer, receiving an EUV scanner equipped with an EUVradiation. The process also includes performing a first exposure processto the substrate, by using the EUV scanner and the EUV mask. The firstexposure process is conducted according to a first exposure dose matrix.The process also includes performing a second exposure process to thesubstrate, by using the EUV scanner and the low EUV reflectivity mask.The second exposure process is conducted according to a second exposuredose matrix.

The present disclosure is also directed towards masks. In oneembodiment, a low EUV reflectivity mask includes a low thermal expansionmaterial (LTEM) layer, a reflective multilayer (ML) having less than 2%EUV reflectivity, which is deposited over the LTEM layer, a cappinglayer over the ML and a patterned absorption layer over the cappinglayer.

In another embodiment, an extreme ultraviolet lithography (EUVL) processto evaluate deep ultraviolet (DUV) flare impacts includes receiving amask pair having a same pattern. The mask pair includes an extremeultraviolet (EUV) mask having a first EUV reflectivity r₁ and a low EUVreflectivity mask having a second EUV reflectivity r₂. The process alsoincludes performing a first exposure process using the EUV mask by anEUV scanner equipped with a EUV radiation to expose a substrate.Exposure doses of the first exposure process are conducted according toa first exposure dose matrix. The first exposure dose matrix includesusing an optimized exposure dose (Eop) of the EUV mask to a first regionof the substrate, using an exposure dose of Eop−Δ to expose a secondregion of the substrate. Here Δ=r₂/r₁×Eop, using an exposure dose ofEop−2Δ to expose a third region of the substrate, . . . , and using anexposure dose of Eop−(N−1)Δ to expose a N region of the substrate. Theprocess also includes performing a second exposure process using the lowEUV reflectivity mask by the EUV scanner having the EUV radiation toexpose the substrate. The exposure doses of the second exposure processare conducted according to a second exposure dose matrix. The secondexposure dose matrix includes using a zero exposure dose on the firstregion of the substrate, using an exposure dose of Eop to expose thesecond region of the substrate, using an exposure dose of 2Eop to exposethe third region of the substrate, . . . , and using an exposure dose of(N−1) Eop to expose the N region of the substrate. The process alsoincludes performing a developing process on the substrate and measuringcritical dimensions (CDs) of the substrate.

The foregoing outlined features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. An apparatus comprising a low EUV reflectivity(LEUVR) mask, wherein the LEUVR mask includes: a low thermal expansionmaterial (LTEM) layer; a reflective multilayer (ML) over the LTEM layer,the reflective ML having less than 2% EUV reflectivity; and a patternedabsorption layer over the reflective ML.
 2. The apparatus of claim 1,wherein the reflective ML includes forty pairs of films and each pair offilms includes a first film having about 1.5 nm molybdenum (Mo) and asecond film having about 2 nm silicon (Si).
 3. The apparatus of claim 1,wherein the reflective ML includes forty pairs of films and each pair offilms includes a first film having about 4.5 nm molybdenum (Mo) and asecond film having about 6 nm silicon (Si).
 4. The apparatus of claim 1,wherein the reflective ML includes about 280 nm molybdenum silicon(MoSi).
 5. The apparatus of claim 1, wherein the LEUVR mask furtherincludes a capping layer between the reflective ML and the patternedabsorption layer.
 6. The apparatus of claim 5, wherein the capping layerincludes about 2.5 nm ruthenium (Ru).
 7. The apparatus of claim 1,further comprising: an extreme ultraviolet (EUV) mask having a EUVreflectivity greater than 60%, wherein the EUV mask and the LEUVR maskhave at least one common pattern.
 8. An apparatus, comprising: anextreme ultraviolet (EUV) mask having a first EUV reflectivity; and alow EUV reflectivity (LEUVR) mask having a second EUV reflectivity,wherein the first EUV reflectivity is greater than the second EUVreflectivity, and wherein the EUV mask and the LEUVR mask have at leastone common pattern.
 9. The apparatus of claim 8, wherein the second EUVreflectivity is less than 2%.
 10. The apparatus of claim 9, wherein thefirst EUV reflectivity is greater than 60%.
 11. The apparatus of claim8, wherein the LEUVR mask comprises: a low thermal expansion material(LTEM) layer; a reflective multilayer (ML) over the LTEM layer, thereflective ML having less than 2% EUV reflectivity; and a patternedabsorption layer over the reflective ML.
 12. The apparatus of claim 11,wherein the reflective ML includes forty pairs of films and each pair offilms includes a first film having about 1.5 nm molybdenum (Mo) and asecond film having about 2 nm silicon (Si).
 13. The apparatus of claim11, wherein the reflective ML includes forty pairs of films and eachpair of films includes a first film having about 4.5 nm molybdenum (Mo)and a second film having about 6 nm silicon (Si).
 14. The apparatus ofclaim 11, wherein the reflective ML includes about 280 nm molybdenumsilicon (MoSi).
 15. The apparatus of claim 11, wherein the LEUVR maskfurther comprises: a capping layer between the reflective ML and thepatterned absorption layer.
 16. The apparatus of claim 15, wherein thecapping layer includes about 2.5 nm ruthenium (Ru).
 17. An apparatus,comprising: an extreme ultraviolet (EUV) mask; and a low EUVreflectivity (LEUVR) mask, wherein the EUV mask includes: a first lowthermal expansion material (LTEM) layer; a first reflective multilayer(ML) over the first LTEM layer, the first reflective ML having a firstEUV reflectivity; and a first patterned absorption layer over the firstreflective ML; wherein the LEUVR mask includes: a second low thermalexpansion material (LTEM) layer; a second reflective multilayer (ML)over the second LTEM layer, the second reflective ML having a second EUVreflectivity; and a second patterned absorption layer over the secondreflective ML; wherein the first EUV reflectivity is greater than thesecond EUV reflectivity; and wherein the first and second patternedabsorption layers have at least one common pattern.
 18. The apparatus ofclaim 17, wherein the first EUV reflectivity is greater than 60% and thesecond EUV reflectivity is less than 2%.
 19. The apparatus of claim 18,wherein the second reflective ML includes forty pairs of films and eachpair of films includes a first film having molybdenum (Mo) and a secondfilm having silicon (Si).
 20. The apparatus of claim 18, wherein thesecond reflective ML includes molybdenum silicon (MoSi).